Urbanization weakens the global latitudinal diversity gradient in birds

The striking increase in species richness toward the tropics is perhaps Earth's most ubiquitous macroecological pattern, but even such fundamental patterns can be vulnerable to anthropogenic change. Urbanization is a particularly severe and globally distributed change that often reduces local diversity; whether such local effects scale up to alter global biodiversity patterns is unclear. We present three hypotheses for how urbanization might affect latitudinal gradients in diversity, and test them using community science observations of >10,000 bird species on five continents. We find that the avian latitudinal diversity gradient is 3× weaker in urban areas compared to natural areas. Urbanization reduced bird diversity across latitudes (by 32% on average), but tropical cities lost a higher proportion of species, an effect that was consistent across hemispheres, seasons, and methodological choices. This disproportionate diversity loss from tropical cities seems driven partly by latitudinal gradients in specialization, as tropical bird assemblages contained more specialists and specialists were disproportionately excluded by urbanization, and partly by more intense urbanization toward lower latitudes. Our results show that urbanization is disrupting macroecological patterns that have defined global biodiversity for millennia, and suggest that both community traits and urban planning mediate urbanization's local and global impacts.